As computing technology has developed, user interface devices have advanced correspondingly. User interfaces have become increasingly significant in the usability of a computing device.
One particular user interface becoming increasingly popular is the touch screen or track pad which uses an array of capacitive sensors using high impedance capacitance substrates. The current, based on the change of the capacitance at the intersection of a row and a column of the array, which varies depending on the presence or absence of a touch, e.g., a finger, etc.
Row and/or columns are scanned sequentially and independently, one by one across the array by a microprocessor. The microprocessor may start by measuring the capacitance at a first column and a first row, then measuring the capacitance for the intersection of the first column and a second row, and then measuring each subsequent intersection in the capacitive sensor array. Thus, if there are 10 rows and 10 columns, a total of 100 measurements of capacitance may be obtained and stored by the microprocessor. Based on the measurements, a centroid corresponding to the finger location may then determined by the microprocessor.
Measuring of each intersection of each row and column may result in the measurements being subject to variations in the physical properties of the sensor array. For example, temperature changes can increase or decrease the capacitance.
Further, measuring capacitance means that the measured range with includes the absolute value of the capacitance. For example, if the capacitance is 8 picofarads (pF) without a finger present and a capacitance of 8.1 pF indicates a touch, the measurement circuit may be calibrated to measure a range of 1 to 10 pF for instance while the dynamic range is only 0.1. This leads to low resolution. The centering of the measurement window by using current compensation may avoid this low resolution. The current compensation involves using a current source to balance out or subtract the base capacitance. The current source is used to provide a current based on the baseline capacitance and thereby subtract out the baseline capacitance from capacitance measurements. The microprocessor accesses and load the baseline values into a programmable current source before each measurement of each row and column intersection. This current compensation solution requires extra hardware which increases costs and is slower as additional operations and settling times increase the time for each scan.
Thus, conventional capacitive sensor systems are susceptible to capacitive variations and utilize ground referenced capacitive measurements resulting in less accurate position information.